1. Field of the Invention
This invention relates generally to the electrodeposition of superconductor compounds and more specifically to the electrodeposition of bismuthates in solution using a constant potentiostatic method and apparatus.
2. Prior Art
Since the discovery of warm superconductivity in the Ba-K-Bi-O system, it has been apparent that chemical approaches to the synthesis of this material must consider its thermal instability. This instability includes phase stability, potassium oxide volatility and the instability of the bismuth oxidation state at high temperatures. Although these problems appear to have been overcome for practical powder synthesis, the low temperature oxidation step usually employed may introduce artifacts into the structure of single crystalline samples, which may in turn make it more difficult to arrive at a theoretical understanding of these superconductors, and to use these superconductors for practical applications.
A maximum oxidation temperature of 450.degree. C. has been suggested by Hinks et al., Synthesis, Structure, and Superconductivity in the Ba.sub.1-x K.sub.x BiO.sub.3-4 System, 333 Nature 836 (1986), based upon thermogravimetric analyses. Progress has been made toward low temperature synthesis of materials in this system using KOH as a flux. The minimum initial growth temperature in these experiments was 360.degree. C. These experiments are usually of long duration (50 hours). This technique, although producing quite useful crystals, is necessarily slow since it depends upon the air (or atmospheric) oxidation of the electroactive element (Bismuth (III)), which will occur at all locations on the melt surface (the ambient/liquid interface). Such slow processes may yield crystals displaying compositional variations reflecting temporal variations in the temperature, concentration and mass transport properties of these high viscosity, molten salts. In fact, a large mosaic spread has been reported for certain of the crystals produced. A crystal grown utilizing a faster growth technique would be expected to display these variations to a lesser extent. Additionally, the corrosive action of these melts may introduce impurities into the system through interaction with the container material.
In Norton, M .L., Electrodeposition of Ba.sub..6 K.sub..4 BiO.sub.3, 24 Mat. Res. Bull. 1391-1397 (1989), deposition control via the use of a reference electrode was not necessary, since constant current gave apparently good results. More detailed analysis shows that the material provided by the potentiostatic method, that is, utilizing constant potential control via use of a novel reference (third) electrode such as Bi metal in contact with Bi ions in solution, is far superior. While the constant current method maintains a constant deposition rate, it does not keep growth per unit of surface area constant. The potentiostatic method does provide constant chemistry, whereas the former method does not. One skilled in the art of electrodeposition would prefer to use constant current, a less complex system than that described herein.
In Maxfield et al., U.S. Patent No. 4,870,051, a method for electrodepositing a mixture of metals suitable for oxidation to form superconducting ceramics is disclosed. This technique does not produce superconductors directly, and gives the indication that the individual elements must be deposited one at a time to make a complex supercondutor. Although it is indicated in the '051 patent that there is potential control, the potentials mentioned do not have chemical meaning, and are not determined in a manner which makes sense of the potentials used in that since the elements are deposited seperately in this technique, the potential used has no direct effect upon the nature of the deposit formed. This is in strong contrast to the method described herein.
In Cuomo et al., U.S. Patent No. 3,498,894, a technique is presented for the synthesis and epitaxial growth of compound semiconductors by fused salt electrolysis. The Cuomo apparatus displays a two electrode cell, without a reference elecrode. Further, two materials are electrochemically active in Cuomo (only one is electroactive at a time in the Maxfield technique), and the potential is set to make the rates of both reactions become equal. In the present invention, materials are produced which will not necessarily have integer ratios of the components. Four different elements are deposited simultaneously, even though only one, Bi, is considered to be electrochemcially active at the potentials used. The three electrode system sets the polymerization rate at a constant value which coincides with the arrival and sticking of relative numbers of Ba and K ions on the surface, followed by incorporation into the bulk by further polymerization. Without this constant polymerization rate, the material will have varying composition as a function of time at growth coordinate. Constant current does not keep the polymerization rate constant, thus the materials formed are graduated, or inhomogeneous. The reason for the potential set herein is intrinsically different from the reason for the potential set in Cuomo.
Few materials with four elements, such as barium, potassium, bismuth and oxygen, have ever been electrodeposited. Since barium, potassium and oxygen are not electroactive in this situation, the change in the bismuth oxidation state can be used to drive crystal growth. Additionally, the use of potassium hydroxide or rubidium hydroxide as an electrochemical solvent is uncommon, and the combination of molten hydroxide chemistry coupled with bismuth is even more uncommon. The closest technology to the present invention is the preparation of small, randomly located crystals in a slowly cooling solution. However, unlike the prior art, the present invention leads to the deposition of bismuth on conducting charged electrodes.
The goal of the crystal growth of nonstoichiometric materials such as those in the present system is to produce large single crystalline deposits or regions of uniform chemical potential, preferrably of any practical nominal composition. Since chemical potential (metal oxidation state) in this system is determined by stoichiometry, an isothermal (chemical potential is normally a function of temperature) technique of electrocrystallization should achieve this goal. In this invention, the preparation of Ba .sub..6 K.sub..4 BiO.sub.3 is used as an example for the demonstration of a novel electrochemical synthetic technique for producing oxidized pseudo-binary oxide materials.